Powders which have to be supplied in precise, measured portions or metered quantities are processed in the pharmaceutical sector, for example, but also in the field of nutritional supplements or the like. Target vessels in the form of blister packs, two-piece capsules, or the like, for example, are filled with measured metered quantities of this kind of a powder product so that the consumer has available and is able to take, corresponding unit doses.
Powder products of this kind are, in particular, conveyed to so-called roller metering devices in individually measured metered quantities which are then decanted into target vessels which have been assigned in each case. A roller metering device of this kind includes a metering roller with a roller jacket which is provided with at least one, usually with a plurality of, metering openings on the circumference, wherein the metering openings are delimited on the inside via a filter element. The filter element is pressed by a clamping core via a filter strip against the inner side of the roller jacket. A pressure channel is arranged in the filter strip, via which the metering opening can be exposed to a negative pressure through the filter element.
Powder is drawn into the metering openings under the action of the negative pressure with metered quantities of the powder being formed, the volume whereof corresponding to the volume of the respective metering opening. The metered quantities formed in such a manner are then ejected from the metering openings and relayed to the target vessel.
It is clear from the preceding comments that the metering which involves a roller metering device is volumetric metering. What is usually the objective, however, is metering in which the measured metered volume has a particular mass within a permitted tolerance range. It has been shown in practice that the metered quantities supplied volumetrically by a roller metering device do not always meet the requirements in terms of the mass actually achieved. Particularly when metering powder products with poor flow properties there are in some cases large deviations from the target mass of the volumetrically supplied metered quantity and therefore a large dispersion of metered masses.
With simple embodiments in the prior art, the filter elements are clamped directly between the filter strip and the roller jacket without additional seals. Sealing elements which are positioned on the side of the filter strip facing the filter elements are used when there are greater requirements. In a configuration known in the art, a seal of this kind is created by a self-adhesive foam pad which is adhered to the filter strip. Such pads are only of limited durability. As time passes, the pads lose elasticity and therefore have to be replaced or removed and disposed of. Alternatively, silicon rings are used in the state of the art, for which complex mounting and positioning aids and also licensed silicon adhesives are required.